A honeycomb structure is widely known as a catalyst carrier for purifying exhaust gas discharged from various internal combustion engines, such as automobile exhaust gas, or a catalyst carrier for deodorizing. In such a honeycomb structure, partition walls partitioning cells have generally been formed linearly over the whole honeycomb structure conventionally in the case of being used for a catalyst for purifying automobile exhaust gas.
On the other hand, in recent years, according to strengthening of an exhaust gas regulation with environmental problems as its background, great importance has been placed on purification ability regarding an automobile gas purification catalyst. Therefore, development of engines for reducing a discharge amount of harmful matter such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxide (NOx) and improvement of three way catalysts for a gasoline engine, which is the main current at present, are making progress, and a discharge amount of harmful matter tends to decrease due to effects by both of them.
Thus, an exhaust amount as a whole upon engine operation has been reduced. On the other hand, an amount of harmful matter discharged right after engine start-up has been high-lighted. For example, in a FTP-75 cycle, which is a U.S. regulated traveling cycle, 60 to 80% of the total discharge amount in the whole traveling cycle is discharged in a cold-transient mode for 140 seconds right after engine start-up.
One of the reasons is that the catalyst is not activated sufficiently because temperature of exhaust gas is low particularly right after engine start up, and, as a result, harmful matter passes the catalyst without being removed. In addition, another reason is that since combustion conditions of fuel is not stable right after engine start-up, A/F (an air/fuel ratio) of exhaust gas, which is an important factor influencing purification performance of a three way catalyst, that is, an oxygen ratio in exhaust gas fluctuates.
Therefore, a catalyst is disposed as close to the engine as possible in a place where exhaust gas temperature is high in order to quickly raise temperature of a catalyst right after engine start-up, or a cell partition wall is thinned in order to lower a heat capacity of the honeycomb carrier itself. In addition, a cell density of the carrier is increased in order to quickly absorb heat of exhaust gas and enlarge a contact area of the catalyst and exhaust gas. There is a problem of increase in pressure loss due to increase in cell density. Further, in the case that an amount of catalyst to be loaded is increased in order to enhance purification ability, it requires a large amount of platinum, which is an expensive catalyst component, or the like, and results in not only increase in cost but also increase in pressure loss because a width of a cell, where exhaust gas actually passes, becomes narrower.
Recently, improvement in mileage has further been required because of the problem of global warming by CO2, and to reduce a pressure loss of a honeycomb structure is more strictly required than before. Therefore, achieving both improvement in purification ability and reduction in pressure loss is desired. In addition, in recent years, a diesel engine is appreciated again because of its low mileage. However, a diesel engine discharges far more particulate matter than a gasoline engine and has a characteristic of trade-off that nitrogen oxide (NOx) increases when the engine is controlled so as to reduce a PM discharge amount. In addition, there arises a problem that nitrogen oxide (NOx) cannot be subjected to a reduction treatment because the exhaust gas has such a high oxygen content that a three way catalyst cannot function as the case of exhaust gas from a gasoline engine. Therefore, a filter using a honeycomb structure is used for removing particulate matter in diesel exhaust gas. However, there arises a problem that a pressure loss becomes high because it has a structure where cell openings are alternately plugged and particulate matter is trapped by the filter partition walls.
On the other hand, it is disclosed to plan uniformalization of exhaust gas flow by changing cell density between the central portion and the outer peripheral portion of a honeycomb structure (see, for example, Patent Documents 1 to 3). For example, Patent Document 1 discloses uniformalization of exhaust gas flow by raising cell density in the central portion. However, there is an apprehension of reduction in thermal shock resistance since there is a discontinuation in a cell structure at a boundary between a portion having high cell density and a portion having low cell density. In addition, a special extrusion die is necessary to prepare such a structure by extrusion forming.
Patent Document 1: U.S. Pat. No. 3853485
Patent Document 2: JP-U-145216
Patent Document 3: JP-U-47310